Imaging systems are limited in terms of image quality by artifacts introduced by the environment that they are operated in. One way to avoid capturing or creating images that have such artifacts is to calibrate the imaging system in an environment that is similar to the environment in which it will be used. Often this is done when the imaging device is manufactured. For example, it is known in the art to use test fixtures to calibrate autofocus systems in film cameras while such cameras are within an operating range of environmental conditions.
In particular, one aspect of an imaging system that benefits from calibration is the autofocus system in an imaging system. Many film cameras, digital cameras and scanners capture images using an imager and a lens system with an adjustable focus lens system. Typically, the focus distance of such an adjustable focus lens system can automatically be set to one of a plurality of different settings by sensing, control and drive systems that are adapted to provide optimal focus of what is determined to be a subject area in a scene. Lens systems that have automatically adjustable focus settings are referred to herein as autofocus systems.
It will be appreciated that it is important to properly calibrate such autofocus systems. In the above example, focus settings for film cameras are calibrated by using the test fixture to monitor an image provided by the lens system of such a film camera and adjusting the lens system until the lens system reaches a first setting where a test target located at a first distance from the camera is in focus. The rangefinder for the film camera is then used to measure the distance to the test target and thereafter the rangefinder will position the lens system at the first setting whenever the rangefinder measures that distance. This process is then repeated for a plurality of other test targets, each located at one of a range of additional distances so that the rangefinding measurements are associated with each of a plurality or lens focus settings.
Digital cameras typically use one of two types of autofocus systems: rangefinder type autofocus systems or a “through-the-lens” type autofocus system to automatically determine taking lens focus settings. A rangefinder autofocus system uses sensors such as optical rangefinders or sonic rangefinders to determine a distance from a camera to one or more portions of a scene within a field of view of the adjustable lens system. Common rangefinder type autofocus systems include active and passive systems. In one example of an active rangefinder type system, the rangefinder type autofocus system compares two low-resolution images that have been captured through two lens systems that are separated laterally by a distance and determine the distance to the scene through triangulation. The focus setting of the adjustable focus lens system is then determined using a calibrated preprogrammed curve or look-up table that correlates scene distances with lens positions that can be used to capture objects at the scene distance in focus. A wide variety of rangefinder type autofocus systems are very well known in the art.
Rangefinder type autofocus systems have the advantage of being very fast with some having a response time that can be in the range of 0.01-0.05 second. However, the focus quality produced by some rangefinder type autofocus systems can vary when they are used in different operating conditions. For example, temperature and humidity can affect the calibration of the distance to focus lens position curve due to fluctuations in the refractive index and dimensions of both the rangefinder autofocus system components and the taking lens components.
The “through-the-lens” autofocus system determines focus settings using analysis of a series of images captured with the lens system positioned at a plurality of different focus distances. For example, in a contrast based “through-the-lens” autofocus system a plurality of different images (e.g. 5-20) are captured with the taking lens in different focus lens positions in a so-called hill climb method. The contrast present in the captured images is compared and the image with the greatest contrast is determined to be the image with the best focus conditions (often the best focus lens position is further refined by interpolating the contrast values between images). The “through-the-lens” type autofocus system is very accurate since it measures focus quality directly from images captured with the high quality taking lens.
However, conventional “through-the-lens” type autofocus systems can be relatively slow in determining a focus setting. For example, such systems can take as long as 0.5-2.0 seconds to determine a focus distance. This is because such “through-the-lens” autofocus systems require the capture and processing of a number of images.
Accordingly, in some digital cameras, the two types of autofocus systems are used together in a hybrid system in which the rangefinder type autofocus system is used to provide a fast estimation of a focus setting that is then followed by the use of the “through-the-lens” autofocus system to refine the focus setting. For example, U.S. Pat. No. 6,864,474 entitled “Focusing Apparatus for Adjusting Focus of an Optical Instrument”, filed by Misawa on Jan. 10, 2003, describes the coordinated use of a rangefinder type autofocus system with a through-the-lens autofocus system. In Misawa, the focus position of the taking lens is determined by both the rangefinder based autofocus system and the “through-the-lens” autofocus system, the difference between the focus position determined by the rangefinder type autofocus system and the focus position determined by the “through-the-lens” type autofocus system is stored for future reference. In subsequent image capture episodes, the stored difference information is used to refine the number of images captured and analyzed by the “through-the-lens” type autofocus system in the hill climb method to determine the focus lens position with best focus, thereby reducing the number of images captured and processed when the rangefinder has been accurate and increasing the number of images captured and processed when the rangefinder has been inaccurate. However, the method described by Misawa assumes that the performance of the rangefinder, adjustable focus lens system, and control system are consistent over time, do not fluctuate with variations in environmental conditions and do not otherwise change or drift over time.
Misawa also does not eliminate the use of multiple image capture and processing used by the “through-the-lens” type autofocus system so that the hybrid autofocus as described by Misawa remains slow. A further aspect of an imaging system that would benefit from calibration is a projection system in order to ensure that a projection lens system is properly focused. There have been efforts to provide automatic feedback systems to this end. For example, U.S. Patent Application Publications US2005/0168705 and US2005/0024606 both by Li et al., describe projection systems which include feedback of a projected image by an imaging sensor system. In this case Li et al. teaches the use of the imaging sensor system to aid in focusing the projector. Li et al. also teaches the use of an imaging sensor system to enable the projection system to correct for projector-to-surface orientation problems, such as correcting to adjust for keystone in the projected image, or to fit the projected image within the edge of a projection screen. Thus, Li et al. discloses, essentially, a “through-the-lens” focus system with orientation compensation. However, here again calibration of such a system is typically performed only during manufacturing or during a manual service procedure.
Therefore the need persists to improve imaging systems through new calibration approaches.